vectored DSL techniques have gained interests in recent years as the vectoring technology can cancel out Far-End CrossTalk (FEXT) basically by using a precoder in downstream and/or a postcoder in upstream. The precoder and/or postcoder will compensate for the effects of crosstalk on a communication channel formed by a group of Digital Subscriber Lines (DSL). When a new line is introduced which needs to join an operative vectoring group (a joining line hereinafter), the precoder and/or postcoder coefficients (or vectoring coefficients hereinafter) have to be updated.
With vectoring, the initialization time of a DSL line has increased from ˜30 seconds to ˜2 minutes. This is often considered unacceptable by operators, since the DSL service will be interrupted by that time when a line retrains. A technique to reduce initialization times is to use historical vectoring coefficients. However, there is a risk that the historic vectoring coefficients are corrupted or not applicable anymore in the next initialization phase. In that case, the line will not come into show-time, and other lines could retrain such that the amount of time spent for the initialization is a multiple of 2 minutes.
Secondly, during tracking, new vectoring coefficients are being computed. However, when applying an updated and not optimal set of vectoring coefficients, vectored lines sharing the same binder may see errors until seamless Rate Adaption (SRA), emergency rate adjustment (SOS) or bit swap kicks in. This may lead to instabilities in a vectored system.